All U.S. patents cited below are herein fully incorporated by reference.
As used herein, the term “thermoplastic” is intended to mean a polymeric material that will melt upon exposure to sufficient heat but will retain its solidified state, but not prior shape without use of a mold or like article, upon sufficient cooling. Specifically, as well, such a term is intended solely to encompass polymers meeting such a broad definition that also exhibit either crystalline or semi-crystalline morphology upon cooling after melt-formation. Particular types of polymers contemplated within such a definition include, without limitation, polyolefins (such as polyethylene, polypropylene, polybutylene, and any combination thereof), polyamides (such as nylon), polyurethanes, polyesters (such as polyethylene terephthalate), and the like (as well as any combinations thereof).
Thermoplastics have been utilized in a variety of end-use applications, including storage containers, medical devices, food packages, plastic tubes and pipes, shelving units, and the like. Such base compositions, however, must exhibit certain physical characteristics in order to permit widespread use. Specifically within polyolefins, for example, uniformity in the orientation of polymer chains upon crystallization is a necessity to provide an effective, durable, and versatile polyolefin article. In order to achieve such desirable physical properties, it has been known that certain compounds and compositions provide nucleation sites for polyolefin crystal growth during molding or fabrication. Generally, compositions containing such nucleating compounds crystallize at a much faster rate than unnucleated polyolefin. Such crystallization at higher temperatures results in reduced fabrication cycle times and a variety of improvements in physical properties, such as, for example, stiffness.
Such compounds and compositions that provide faster and/or higher polymer crystallization temperatures are thus popularly known as nucleators. Such compounds are, as their name suggests, utilized to provide nucleation sites for crystal growth during cooling of a thermoplastic molten formulation. Generally, the presence of such nucleation sites results in a larger number of smaller crystals. As a result of the smaller crystals formed therein, clarification of the target thermoplastic may also be achieved, although excellent clarity is not always a result. The more uniform, and preferably smaller, the crystal size, the less light is scattered. In such a manner, the clarity of the thermoplastic article itself can be improved. Thus, thermoplastic nucleator compounds are very important to the thermoplastic industry in order to provide enhanced clarity, physical properties and/or faster processing.
As an example, dibenzylidene sorbitol derivatives are common nucleator compounds, particularly for polypropylene end-products. Compounds such as 1,3-O-2,4-bis(3,4-dimethylbenzylidene) sorbitol (hereinafter DMDBS), available from Milliken Chemical under the trade name Millad® 3988, provide excellent nucleation and clarification characteristics for target polypropylenes and other polyolefins. Other well known nucleator compounds include sodium benzoate, sodium 2,2′-methylene-bis-(4,6-di-tert-butylphenyl) phosphate (from Asahi Denka Kogyo K.K., known as NA-11), aluminum bis[2,2′-methylene-bis-(4,6-di-tert-butylphenyl)phosphate] (also from Asahi Denka Kogyo K.K., known as NA-21), talc, and the like. Such compounds all impart high polyolefin crystallization temperatures; however, each also exhibits its own drawback for large-scale industrial applications.
For example, of great interest is the compatibility of such compounds with different additives widely used within typical polyolefin (e.g., polypropylene, polyethylene, and the like) plastic articles. For instance, calcium stearate is a very popular acid neutralizer present within typical polypropylene formulations to protect the stabilizing additives (such as light stabilizers, antioxidants, etc.) from catalyst residue attack. Unfortunately, most of the nucleator compounds noted above also exhibit deleterious reactions with calcium stearate within polyolefin articles. For sodium, and other like metal ions, it appears that the calcium ion from the stearate transfers positions with the sodium ions of the nucleating agents, rendering the nucleating agents ineffective for their intended function. As a result, such compounds sometimes exhibit unwanted plate-out characteristics and overall reduced nucleation performance (as measured, for example, by a decrease in crystallization temperature during and after polyolefin processing). Other processing problems are evident with such compounds as well.
Other problems encountered with the standard nucleators noted above include inconsistent nucleation due to dispersion problems, resulting in stiffness and impact variation in the polyolefin article. Substantial uniformity in polyolefin production is highly desirable because it results in relatively uniform finished polyolefin articles. If the resultant article does not contain a well-dispersed nucleating agent, the entire article itself may suffer from a lack of rigidity and low impact strength.
Furthermore, storage stability of nucleator compounds and compositions is another potential problem with thermoplastic nucleators and thus is of enormous importance as well. Since nucleator compounds are generally provided in powder or granular form to the polyolefin manufacturer, and since uniform small particles of nucleating agents is imperative to provide the requisite uniform dispersion and performance, such compounds must remain as small particles through storage. Certain nucleators, such as sodium benzoate, exhibit high degrees of hygroscopicity such that the powders made therefrom hydrate easily resulting in particulate agglomeration. Such agglomerated particles may require further milling or other processing for deagglomeration in order to achieve the desired uniform dispersion within the target thermoplastic. Furthermore, such unwanted agglomeration due to hydration may also cause feeding and/or handling problems for the user.
These noticeable problems have thus created a long-felt need in the thermoplastic industry to provide nucleating/clarifying agents that do not exhibit the aforementioned problems and provide excellent peak crystallization temperatures for the target thermoplastics themselves, particularly with a wide variety of typical and necessary acid scavenger additives. To date, the best compounds for this purpose remain those noted above. Unfortunately, nucleators exhibiting exceptionally high peak crystallization temperatures, low hygroscopicity properties, excellent dispersion and concomitant clarity and stiffness, as well as compatibility with most standard polyolefin additives (such as, most importantly, calcium organic salt acid scavengers) have not been accorded the different thermoplastic industries. Such problems are not limited to polyolefins and are common within all thermoplastic applications in which nucleating agents are used.
Of greater particular concern and interest is the effect nucleators impart on polypropylene resins and articles made therefrom. Polypropylene is a semi-crystalline polymer valued for its low cost, low density, chemical resistance, and excellent stiffness but it suffers from inferior impact resistance. Polypropylene can be modified by a number of different routes to influence the final physical properties of the resin, especially the stiffness-impact balance. Generally, impact resistance can be improved by incorporation of a rubber phase into the polypropylene by either melt compounding or copolymerisation of other olefinic monomers such as ethylene, butylene or higher order olefins. These rubbery components phase segregate in these impact copolymer polypropylenes forming energy absorbing domains and increase the impact resistance of the resin at room temperature as well as very low temperatures, down to −30° C. or lower depending on the rubber content. However, these impact copolymers, which are widely available from most commercial polypropylene producers, suffer from significantly reduced strength and stiffness. Therein lies the problem with polypropylene; there is a fundamental trade-off between having high stiffness, which tends to decrease the impact resistance, and high impact, which tends to decrease the resin stiffness.
To overcome this issue, filling agents are used to modify the physical properties of polypropylene to increase stiffness. For example U.S. Pat. No. 6,156,836 describes a composite material comprised of highly specific combinations of polypropylene, rubber components (such as ethylene-propylene copolymer), and talc filler at 15–25% by weight in order to create a resin with improved stiffness-impact balance arising from a specific “b-axis” orientation of the crystals within molded parts. Furthermore U.S. Pat. No. 5,591,795 also describes a composite material comprised of highly specific combinations of polypropylene, rubber components (such as ethylene-propylene copolymer), and talc filler at 5–15% by weight, also for stiffness-impact benefits. The talc components in these inventions impart rigidity to the sample because of its inorganic nature and its ability to induce a highly specific “b-axis” orientation into the polypropylene. “B-axis” orientation occurs when the polypropylene crystals align in a specific orientation within the part such that the crystallographic “b-axis” of the polypropylene crystals aligns with the normal, or thickness, direction of the molded part, while the a and c axes are distributed about the machine and transverse directions.
However, such a method of blending together these components (polypropylene, rubber, filler) is not easily accomplished in every situation and suffers from the drawbacks of creating increased costs for the resin from the extra compounding steps needed to get thorough mixing and dispersion of all of the components, increased polymer density from the large mass fraction of inorganic fillers, loss of impact resistance, and furthermore these materials are completely opaque. An improved method of obtaining this effect would be to have a low density (organic), low usage-level nucleating agent that could impart a similar effect. This additive would primarily have an effect on the hompolymer fraction of the blend, either in an impact copolymer or a random copolymer. As evinced by the degrees to which the industry is seeking a solution to this stiffness-impact conundrum through the use of complex blends, the invention described herein can be used to overcome this problem.